Re: [Lilypond-auto] Issue 4083 in lilypond: \keepWithTag does not nest well

[lilypond]

Comment #4 on issue 4083 by address@hidden: \keepWithTag does not nest  
well
https://code.google.com/p/lilypond/issues/detail?id=4083

Tags are a useful concept.  The limited logical operations we have on tags  
makes using them a frustrating exercise in Venn diagrams.
\removeWithTag is useful. \keepWithTag causes trouble when we change the  
set of tags, or add an orthogonal set, mostly because \keepWithTag leaves  
the matching tags on its output.

We can add potentially better tag-operations, and let people testing the  
development branch see how well they work in practice.

(a) \keepWithTag A  would need to fall-back to its old behavior of removing  
music with non-matching tags, if the tag A is not declared in any  
\tagLayer, so that old examples work:
 << {s2*4\p s2*4\>}
    \repeat unfold 8{ ef8 r \tuplet3/2{ ef ef ef } }
    \tag #'part {\repeat percent 8 s2} >>

Making a new function \keepFromLayerOnlyTag lets us try the new method more  
freely. (The global scope of the layer definitions gives me a nagging  
worry, as well.)

(c) has a simple version: \keepWithTag A returns the music that was tagged  
A, but with the tag removed so it does not interfere with later tag-filters.

Tag removal is useful on its own.  If the filtering operations on a  
tagLayer are reasonably confined, such as within a music function, we can  
clean up that tag layer after use, without defining the layer globally.  If  
A.B.C.D is our tagLayer,
 \clearTags A \keepWithTag A    % maybe \keepWithTag should have done this  
all along
 \clearTags A \removeWithTag B.C.D % transparent to other tags, so better  
than above
 \clearTags B.C.D \removeWithTag A

(b) looks *very* tempting, because it seems to work so intuitively, without  
requiring a separate declaration of the \tagLayer.  In both examples of  
comment #2 it does what I would have expected a tag-selector function to do.
"\chooseWithTag A removes music with tags other than A that are found  
alongside music tagged A in a parallel or sequential construct."  Probably  
\chooseWithTag A should remove tag-A from its output.

Attachments:
        cleartags.ly  473 bytes

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